I'm "little" skeptical about effectiveness of this (it affect handling caching filesystem vs. application data in RAM, and RAM use same ammount of power, no matter if You got 200 used or 200 free...), at least, unless someone may provide hard proof via - for example - 'bq27200.sh 5' current monitoring of ~30 minutes in idle, in same N900 with only this setting changed. AFAIK, no one proved real benefit here, so I may be wrong.

Still, thanks for Your work, monitoring dbus signals may be useful in other cases

Ps.

don't set limits to:
... it simply won't work. With 125 and 250 blacklisted, device will still idle @ 500 - so, no harm, no gain - and without blacklisting 125 and 250, it may lock on some of them, under specific conditions.

"0" isn't proper value for CPU frequency. CPU underclock to effective 0 mhz via other mechanism (when idle), not controllable via kernel settings, at least currently.

If someone really want, limiting it to "500 500" will work, but device with limits 500-900 doesn't use more power in idle. In fact, when I measured it limited to 500 vs. limited to 900 for ~10 hours, limits on 900 used less power - ho ever, only by 30-40 mAh (during ~10 hours, not per hour).